Method of making powder metal parts by surface densification

ABSTRACT

A method of producing parts from powdered metal having a first step of providing a powder, which is compressed at a pressure of 25 to 65 tsi to provide a green compact. The compact is then sintered at 2000° F. to 2400° F. for 20 to 60 minutes and cooled. After the compact has been cooled, the density of the compact is increased to greater than 7.4 g/cc. The compact is austenitized in an atmosphere of gas containing propane and a carbon potential of 0.8%. In one embodiment, the gas also contains ammonia. The compact is heated in the atmosphere at a temperature of 1600° F. for 40 minutes. Immediately following the heating, the compact is quenched in oil a temperature between 120° F. and 150° F. for 10 to 15 minutes. Lastly the compact is tempered at a temperature between 300° F. and 1000° F. for 30 to 90 minutes.

REFERENCE TO RELATED APPLICATIONS

The subject matter of this application is related to copending application Ser. No. 10/983,554 filed Nov. 8, 2004, entitled “METHOD OF PRODUCING POWDER METAL PARTS” which is a continuation-in-part of parent application Ser. No. 10/697,344 filed Oct. 30, 2003 entitled, “METHOD OF PRODUCING POWDER METAL PARTS” which claims the benefit of provisional application No. 60/432,823, filed Dec. 12, 2002, entitled “METHOD OF PRODUCING POWDER METAL PARTS”. The aforementioned applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to the field of powder metallurgy. More particularly, the invention pertains to a method of making metal parts by surface densification.

2. Description of Related Art

Various methods have been used to form high performance parts with powder metal. For example, U.S. Pat. No. 5,613,180 discloses a method of forming high density and/or high surface density ferrous powder metal parts by compacting a powder containing iron, 0.8-1.5 wt % Mo, 0.14 wt % Mn, 0.07 wt % O, no greater than 0.01 wt % C or substantially no graphite. The powder is compacted at 40-50 tsi to a density of 7.15-7.25 g/cc. Then the compact is sintered at a temperature of 2050-2300° F. for 30-60 minutes. The sintered compact is then repressed at 55-65 ksi to increase the density to 7.55-7.68 g/cc. The sintered compact is heated up from ambient temperature to 400° F. in an endogas environment with a composition of 40 parts by volume hydrogen, 40 parts by volume nitrogen, and 10 parts by volume carbon monoxide, with traces of other gases, for about 60 minutes. Then the carbon potential in the atmosphere is increased to 0.9-1.1% C and the temperature is increased to 1750° F. and held at that temperature for 75 minutes. The compact is then cooled to 1600° F. and the carbon potential of the atmosphere is adjusted to 0.7-0.8% C and held for 150 minutes. The carbon skin formed on the compact is to a depth of at least 0.010 inches. The compact is then cooled to 1500° F. and oil quenched. Lastly, the quenched compact is tempered for 60 minutes at 400° F.

Another example, U.S. Pat. No. 6,143,240, discloses a method of making a high density powdered metal article. A selected powder is compacted under a pressure of 40 tsi. The compacted article is then sintered at 1250° C. The sintered compact is then subject to forming, which includes coining, sizing, repressing or striking. After the compact is formed, the compact is annealed.

U.S. Pat. No. 6,301,101 discloses a method of making metal gears out of powder consisting of 0.2-0.3 wt % C, 0-0.2 wt % S, up to 0.03 wt % P, 0.1-0.25 wt % Mn, 0.5-0.6 wt % Mo, 1.75-2 wt % Ni, 0-0.03 wt % Si, 0-0.1 wt % Cr, 0-0.15 wt % Cu and greater than 50 wt % Fe; pressing the powder at a pressure of 20-70 tsi into a mold resulting in a green density of 6.2-7.2 g/cc; sintering the green compact in a range of 1400-2100° F.; hot forming the sintered compact at a temperature of 1400° F.-2100° F. at a pressure of 20-90 tsi; carburizing the compact at 1400-2400 F; resintering the compact at 1400-2400 F for 10-60 minutes and cooled at a rate suitable to form bainite in one or more surface regions of the preform that may be subjected to shaving; and shaving the compact.

SUMMARY OF THE INVENTION

A method of producing parts from powdered metal having a first step of providing a powder, which is compressed at a pressure of 25 to 65 tsi to provide a green compact. The compact is then heated to 2000° to 2400° F. for 20 to 60 minutes and cooled. After the compact has been cooled, at least a portion the density of the compact is increased to greater than 7.4 g/cc. The compact is then placed in an atmosphere of gas containing propane and a carbon potential of 0.8%. In one embodiment, the gas also contains ammonia. The compact is heated in the atmosphere at a temperature of 1600° F. for 40 minutes. Immediately following the heating, the compact is quenched in oil at a temperature between 120° and 150° F. for 10 to 15 minutes. Lastly the compact is heated to a temperature between 300° and 1000° F. for 30 to 90 minutes

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a block diagram of the steps of a method of the present invention that produces powder metal parts.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram that shows a method of producing powder metal parts. In a first step 101, a mixture of metallurgical powder comprised of iron, 0-1.5 weight percent silicon, 0.15-0.9 weight percent graphite, 0.5-1.0 weight percent molybdenum, 0-4.5 weight percent nickel, 0-0.5 weight percent manganese, 0-2.0 weight percent copper, and 0-4.0 weight percent chromium is mixed together, see Table 1. TABLE 1 Fe Graphite Si Mo Ni Mn Cu Cr Pow- Balance 0.15-0.9 0-1.5 0.5-1.0 0-4.5 0-0.5 0-2.0 0-4.0 der

The second step 102 of the method is to compact or compress the mixture of powders. The powders are compacted with a compaction pressure in the range of 25 to 65 tsi, preferably, 40 to 45 tsi, resulting in a green compact with a green density of 6.4 to 7.4 g/cc.

In the third step 103, the compact is sintered. The term “sintered” throughout this application refers to high temperature sintering or regular sintering and is dependent on the composition of the metallurgic powder being used. For high temperature sintering, the green compact is sintered at 2100° F. to 2400° F. for 20 to 60 minutes, preferably 2300° F. for 30 minutes. For regular sintering, the green compact is sintered at 2000° F. to 2100° F. for 20 to 60 minutes, preferably 2070° F. for 30 minutes.

The carbon content of the powder mixture, is dependent on the amount of graphite present, since graphite becomes sinter carbon after sintering. If the carbon content is greater than 0.4 wt %, then an annealing step 109 occurs prior to surface densification. Annealing is a heat treatment process where the microstructure is modified to lower material hardness and improve formability. Annealing is used to soften metals but does not include any mechanical deformation. Annealing includes heating to and holding at a suitable temperature, followed by cooling at a suitable rate. To anneal the compact, the compact is heated to a temperature of 1450° F. to 1700° F. for 20 to 60 minutes, preferably 1560° F. for 35 minutes. Alternatively, if the carbon content is less than 0.4 wt %, then an annealing step is not necessary. The compact is then cooled to ambient temperature.

Surface densification is carried out by mechanical working in the next step 104. Examples of mechanical working include sizing, rolling, roller burnishing, shot peening or blasting, extruding, swaging, cold forming, and hot forming. At least a portion of the surface area of the sintered compact is increased to greater than 7.4 g/cc.

After densification 104, the compact undergoes carburizing or carbonitriding 105 to increase the surface hardness of the parts. In one embodiment, carburizing may be preferred over carbonitriding due to application requirements, cost, customer demand, and metallurgic powder content. For carburizing, the compact is brought into contact with an atmosphere of preferably 400 cfh of endothermic gas preferably containing 10 cfh of propane and having a carbon potential of 0.4% to 0.9%, preferably 0.8%. The compact is heated in the above atmosphere at a temperature of 1600° F. for 40 minutes. The term “carbon potential” refers to the extent in which an environment contains active carbon that can affect the carbon content of the compact.

Alternatively, in another embodiment carbonitriding is preferred over carburizing, due to application requirements, customer demand, cost, and the metallurgic content of the powder mixture. For carbonitriding, the compact is brought into contact with an atmosphere containing both carbon and nitrogen, which are simultaneously absorbed and diffused into the metal compact. After carbonitriding, the compact has a hard, wear-resistant case that is shallower than the compact that has been carburized. The compact is carbonitrided in an atmosphere of preferably 400 cfh of endothermic gas containing 10 cfh of propane, 10 cfh of ammonia, and a carbon potential of 0.4% to 0.9%, preferably 0.8%. The compact is heated in the above atmosphere at a temperature of 1600° F. for 40 minutes. The term “carbon potential” refers to the extent in which an environment contains active carbon that can affect the carbon content of the compact.

After the compact has undergone carbuzing or carbonitriding, the compact is immediately quenched in oil 106 at a temperature of 120°-150 F for 10 to 15 minutes.

After oil quenching, the compact undergoes a tempering step 107 in which the compact is heated at a temperature of 300°-1000° F. for 30 to 90 minutes. The tempering of the compact is preferably at 400° F. for 60 minutes.

Lastly, a secondary operation 108 may be performed on the compact. Examples of a secondary operation include but are not limited to honing, broching, and deburring.

EXAMPLE 1

A powder including at least 3 wt % Cr and 0.5% wt % Mo was blended with 0.25 wt % graphite. The powders were compacted into slugs with a 25 mm diameter (25 mmOAL) with a green density of 6.95 g/cc, were sintered at 2070° F. for 30 minutes and then cooled to ambient temperature. The slugs were then rolled in a roller machine with three roller dies for surface densification.

After densification, half of the slugs were carburized and placed within a chamber of a Dow Internal Quench Batch type furnace in which the atmosphere of 400 cfh of endothermic gas contained 10 cfh of propane and had a carbon potential of 0.8%. While in this atmosphere, the slugs were heated to a temperature of 1600° F. for 40 minutes. Then, the slugs were quenched in oil at a temperature of 120° F. for 10 minutes and tempered at 400° F. for 60 minutes. Referring to Graph 1, the hardness of the slugs is shown from the outer surface at 0 mm to 0.3 mm. The particle hardness of the slugs decreases from the surface of the slugs to the center of the slugs. The particle hardness at the outer surface is 837 HK 100 and decreases only slightly to 830 HK 100 through 0.1 mm from the outer surface of the slugs. Between 0.1 mm and 0.3 mm, the particle hardness continues to decrease to 650 HK 100, showing that the microstructure at this depth remains to be marten site. The case depth of the carburized slug is 0.97 mm.

The remaining slugs underwent carbonitriding and were placed within a chamber of a Dow Internal Quench Batch Type Furnace manufactured by Dow Furnace Company, in which the atmosphere at 400 cfh of endothermic gas contained 10 cfh of propane, 10 cfh of ammonia, and a carbon potential of 0.8%. While in this atmosphere, the slugs were heated to a temperature of 1600° F. for 40 minutes. Then, the slugs were quenched in oil at a temperature of 120° F. for 10 minutes and tempered at 400° F. for 60 minutes. Referring to Graph 1, the hardness of the slugs are shown from the outer surface at 0 mm to 0.3 mm. The particle hardness of the slugs decreases from the surface of the slugs to the center of the slugs. The particle hardness at the outer surface is 823 HK 100 and decreases only slightly to 813 HK 100 through 0.1 mm from the outer surface of the slugs. Between 0.1 mm and 0.3 mm, the particle hardness continues to decrease to 662 HK 100, showing that the microstructure at this depth remains be martensite.

The surface density of the compact after undergoing carbonitriding or carburization was about 7.88 g/cc, and remained around 7.6 g/cc 0.1 to 0.5 mm from the surface as exhibited in Graph 2, showing the density profile of the slugs at 10× magnification.

EXAMPLE 2

A powder including at least 3 wt % Cr and 0.5% wt % Mo was blended with 0.65 wt % graphite. The powders were compacted into slugs with a 25 mm diameter (25 mmOAL) with a green density of 6.95 g/cc, were sintered at 2070° F. for 30 minutes and then cooled to ambient temperature. The slugs were annealed at 1560° F. for 35 minutes to improve formability due to the high carbon content. The slugs were then rolled in a roller machine with three roller dies for surface densification.

After densification, half of the slugs were carburized and placed within a chamber of a Dow Internal Quench Batch Type Furnace manufactured by Dow Furnace Company in which the atmosphere of 400 cfh of endothermic gas contained 10 cfh of propane and had a carbon potential of 0.8%. While in this atmosphere, the slugs were heated to a temperature of 1600° F. for 40 minutes. Then the slugs were quenched in oil at a temperature of 120° F. for 10 minutes and tempered at 400° F. for 60 minutes. Referring to Graph 1, the hardness of the slugs is shown from the outer surface at 0 mm to 0.3 mm. The particle hardness of the slugs decreases from the surface of the slugs to the center of the slugs. The particle hardness at the outer surface is 864 HK 100 and decreases to 839 HK 100 through 0.1 mm from the outer surface of the slugs. Between 0.1 mm and 0.3 mm, the particle hardness continues to decrease to 800 HK 100, showing that the microstructure at this depth remains to be martensite. The case depth of the carburized slug is 1.25 mm.

The remaining slugs underwent carbonitriding and were placed within a chamber of a Dow Internal Quench Batch Type Furnace manufactured by Dow Furnace Company, in which the atmosphere at 400 cfh of endothermic gas contained 10 cfh of propane, 10 cfh of ammonia, and a carbon potential of 0.8%. While in this atmosphere, the slugs were heated to a temperature of 1600° F. for 40 minutes. Then the slugs were quenched in oil at a temperature of 120° F. for 10 minutes and tempered at 400° F. for 60 minutes. Referring to Graph 1, the hardness of the slugs are shown from the outer surface at 0 mm to 0.3 mm. The particle hardness of the slugs decreases from the surface of the slugs to the center of the slugs. The particle hardness at the outer surface is 867 HK 100 and decreases only slightly to 851 HK 100 through 0.1 mm from the outer surface of the slugs. Between 0.1 mm and 0.3 mm, the particle hardness continues to decrease to 805 HK 100, showing that the microstructure at this depth remains be martensite.

The surface density of the compact after undergoing carbonitriding or carburization was about 7.88 g/cc, and remained around 7.6 g/cc 0.1 to 0.4 mm from the surface as exhibited in Graph 2, showing the density profile of the slugs at 10× magnification.

Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention. 

1. A method of producing parts from powdered metal comprising the steps of: a) providing a metallurgic powder; b) compressing the metallurgic powder at a pressure of 25 tsi to 65 tsi to provide a green compact; c) heating the compact to a temperature of 2000° to 2400° F. for 20 to 60 minutes; d) cooling the compact to ambient temperature; e) increasing the density of at least a portion of the compact to greater than 7.4 g/cc; f) placing the compact in an atmosphere of endothermic gas containing propane and a carbon potential of 0.4% to 0.9%; g) heating the compact in the atmosphere of endothermic gas at a temperature of 1600° F. for 40 minutes; h) quenching the compact in oil heated to a temperature between 120° F. and 150° F. for 10 to 15 minutes; and i) heating the compact to a temperature between 300° F. and 1000° F. for 30 to 90 minutes.
 2. The method of claim 1, wherein the metallurgic powder is comprised of iron, 0.15 to 0.9 weight percent graphite, and 0.5 to 1.0 weight percent molybdenum, wherein the weight percentages are calculated based on a total weight of the powder.
 3. The method of claim 2, wherein the metallurgic powder further comprises up to 0.5 weight percent manganese, wherein the weight percentages are calculated based on a total weight of the powder.
 4. The method of claim 2, wherein the metallurgic powder further comprises up to 1.5 weight percent silicon, wherein the weight percentages are calculated based on a total weight of the powder.
 5. The method of claim 2, wherein the metallurgic powder further comprises up to 4.5 weight percent nickel, wherein the weight percentages are calculated based on a total weight of the powder.
 6. The method of claim 2, wherein the metallurgic powder further comprises up to 2.0 weight percent copper, wherein the weight percentages are calculated based on a total weight of the powder.
 7. The method of claim 2, wherein the metallurgic powder further comprises up to 4.0 weight percent chromium, wherein the weight percentages are calculated based on a total weight of the powder.
 8. The method of claim 1, wherein the green compact of step b) has a green density of 6.4 g/cc to 7.4 g/cc.
 9. The method of claim 1, further comprising heating the compact to a temperature of 1560° F. for 35 minutes between steps d) and e).
 10. The method of claim 1, wherein the step of increasing the density of at least a portion of the compact in step e) is selected from a group consisting of sizing, rolling, roller burnishing, shot peening or blasting, extruding, swaging, cold forming, and hot forming.
 11. The method of claim 1, wherein the atmosphere comprises 400 cfh of endothermic gas and 10 cfh of propane.
 12. The method of claim 11, wherein the atmosphere further comprises 10 cfh of ammonia.
 13. The method of claim 1, wherein the compact in step i) is heated to a temperature of 400° F. for 60 minutes.
 14. The method of claim 1, wherein the compact is not cooled below the temperature of step f) between steps f) and g).
 15. The method of claim 1, wherein the compact in step c) is heated to a temperature of 2000° F. to 2100° F. for 20 to 60 minutes.
 16. The method of claim 1, wherein the compact in step c) is heated to a temperature of 2070° F. for 30 minutes.
 17. The method of claim 1, wherein the compact in step c) is heated to a temperature of 2100° F. to 2400° F. for 20 to 60 minutes.
 18. The method of claim 1, wherein the compact in step c) is heated to a temperature of 2300° F. for 30 minutes.
 19. A method of producing parts from powdered metal comprising the steps of: a) providing a metallurgic powder; b) compressing the metallurgic powder at a pressure of 25 tsi to 65 tsi to provide a green compact; c) heating the compact to a temperature of 2000° F. to 2400° F. for 20 to 60 minutes; d) cooling the compact to ambient temperature; e) increasing the density of at least a portion of the compact to greater than 7.4 g/cc; f) placing the compact in an atmosphere of endothermic gas containing propane, ammonia, and a carbon potential of 0.4% to 0.9%; g) heating the compact in the atmosphere of endothermic gas at a temperature of 1600° F. for 40 minutes; h) quenching the compact in oil heated to a temperature between 120° F. and 150° F. for 10 to 15 minutes; and i) heating the compact to a temperature between 300° F. and 1000° F. for 30 to 90 minutes.
 20. The method of claim 19, wherein the metallurgic powder is comprised of iron, 0.15 to 0.9 weight percent graphite, and 0.5 to 1.0 weight percent molybdenum, wherein the weight percentages are calculated based on a total weight of the powder.
 21. The method of claim 20, wherein the metallurgic powder further comprises up to 0.5 weight percent manganese, wherein the weight percentages are calculated based on a total weight of the powder.
 22. The method of claim 20, wherein the metallurgic powder further comprises up to 1.5 weight percent silicon, wherein the weight percentages are calculated based on a total weight of the powder.
 23. The method of claim 20, wherein the metallurgic powder further comprises up to 4.5 weight percent nickel, wherein the weight percentages are calculated based on a total weight of the powder.
 24. The method of claim 20, wherein the metallurgic powder further comprises up to 2.0 weight percent copper, wherein the weight percentages are calculated based on a total weight of the powder.
 25. The method of claim 20, wherein the metallurgic powder further comprises up to 4.0 weight percent chromium, wherein the weight percentages are calculated based on a total weight of the powder.
 26. The method of claim 19, wherein the green compact of step b) has a green density of 6.4 g/cc to 7.4 g/cc.
 27. The method of claim 19, further comprising heating the compact to a temperature of 1560° F. for 35 minutes between steps d) and e).
 28. The method of claim 19, wherein the step of increasing the density of at least a portion of the compact in step e) is selected from a group consisting of sizing, rolling, roller burnishing, shot peening or blasting, extruding, swaging, cold forming, and hot forming.
 29. The method of claim 19, wherein the atmosphere comprises 400 cfh of endothermic gas and comprises 10 cfh of propane.
 30. The method of claim 19, wherein the compact in step i) is heated to a temperature of 400° F. for 60 minutes.
 31. The method of claim 19, wherein the compact is not cooled below the temperature of step f) between steps f) and g).
 32. The method of claim 19, wherein the compact in step c) is heated to a temperature of 2000° F. to 2100° F. for 20 to 60 minutes.
 33. The method of claim 19, wherein the compact in step c) is heated to a temperature of 2070° F. for 30 minutes.
 34. The method of claim 19, wherein the compact in step c) is heated to a temperature of 2100° F. to 2400° F. for 20 to 60 minutes.
 35. The method of claim 19, wherein the compact in step c) is heated to a temperature of 2300° F. for 30 minutes. 